An ion source is a device that causes gas molecules to be ionized and then accelerates and emits the ionized gas molecules and/or atoms in a beam toward a substrate. Such an ion beam may be used for various technical and technological purposes, including but not limited to, cleaning, activation, polishing, etching, and/or deposition of thin film coatings. Exemplary ion sources are disclosed, for example, in U.S. Pat. Nos. 6,037,717; 6,002,208; and 5,656,819, the disclosures of which are all hereby incorporated herein by reference.
FIGS. 1 and 2 illustrate a conventional ion source. In particular, FIG. 1 is a side cross-sectional view of an ion beam source with a circular ion beam emitting slit defined in the cathode, and FIG. 2 is a corresponding sectional plan view along section line II—II of FIG. 1. FIG. 3 is a sectional plan view similar to FIG. 2, for purposes of illustrating that the FIG. 1 ion beam source may have an oval ion beam emitting slit as opposed to a circular ion beam emitting slit.
Referring to FIGS. 1-3, the ion source includes hollow housing 3 made of a magnetoconductive material such as steel, which is used as a cathode 5. Cathode 5 includes cylindrical or oval side wall 7, a closed or partially closed bottom wall 9, and an approximately flat top wall 11 in which a circular or oval ion emitting slit 15 is defined. Ion emitting slit 15 includes an inner periphery 17 as well as an outer periphery 19.
Deposit and maintenance gas supply aperture or hole 21 is formed in bottom wall 9. Flat top wall 11 functions as an accelerating electrode. A magnetic system in the form of a cylindrical permanent magnet 23 with poles N and S of opposite polarity is placed inside housing 3 between bottom wall 9 and top wall 11. The N-pole faces flat top wall 11, while the S-pole faces bottom wall 9 of the ion source. The purpose of the magnetic system, including magnet 23 with a closed magnetic circuit formed by the magnet 23, cathode 5, side wall(s) 7, and bottom wall 9, is to induce a substantially transverse magnetic field (MF) in an area proximate ion emitting slit 15.
A circular or oval shaped anode 25, electrically connected to positive pole 27 of electric power source 29, is arranged in the interior of housing 3 so as to at least partially surround magnet 23 and be approximately concentric therewith. Anode 25 may be fixed inside the housing by way of ring 31 (e.g., of ceramic). Anode 25 defines a central opening 33 therein in which magnet 23 is located. Negative pole 35 of electric power source 29 is connected to housing 3 (and thus to cathode 5) generally at 37, so that the cathode and housing are grounded (GR).
Located above housing 3 (and thus above cathode 5) of the ion source of FIGS. 1-3 is vacuum deposition chamber 41. Chamber 41, used for depositing, includes evacuation port 43 that is connected to a source of vacuum (not shown). An object or substrate 45 to be treated (e.g., coated, etched, cleaned, etc.) is supported within vacuum chamber 41 above ion emitting slit 15 (e.g., by gluing it, fastening it, or otherwise supporting it on an insulator block 47). Thus, substrate 45 can remain electrically and magnetically isolated from the housing of vacuum depositing chamber 41, yet electrically connected via line 49 to negative pole 35 of power source 29. Since the interior of housing 3 can communicate with the interior of vacuum depositing chamber 41, all lines that electrically connect power source 29 with anode 25 and substrate 45 may pass into the interior of housing 3 and/or chamber 41 via conventional electrically feed through devices 51.
The conventional ion beam source of FIGS. 1-3 is intended for the formation of a unilaterally directed tubular ion beam 53, flowing in the direction of arrow 55 toward a surface of substrate 45. Ion beam 53 emitted from the area of slit 15 is in the form of a circle in the FIG. 2 embodiment and in the form of an oval (i.e., race track) in the FIG. 3 embodiment.
The ion beam source of FIGS. 1-3 operates as follows. Vacuum chamber 41 is evacuated, and a depositing gas 57 is fed into the interior of housing 3 via aperture 21. Power supply 29 is activated and an electric field is generated between anode 25 and cathode 5, which accelerates electrons 59 to high energy. Electron collisions with the gas in or proximate gap or slit 15 leads to ionization and a plasma is generated. “Plasma” herein means a cloud of gas including ions of a material to be accelerated toward substrate 45. The plasma expands and fills (or at least partially fills) a region including slit 15. An electric field is produced in slit 15, oriented in the direction of arrow 55 (substantially perpendicular to the transverse magnetic field) which causes ions to propagate toward substrate 45. Electrons in the ion acceleration space in slit 15 are propelled by the known E×B drift in a closed loop path within the region of crossed electric and magnetic field lines proximate slit 15. These circulating electrons contribute to ionization of the gas, so that the zone of ionizing collisions extends beyond the electrical gap 63 between the anode and cathode and includes the region proximate slit 15 on one and/or both sides of the cathode 5.
For purposes of example, consider the situation where a silane and/or acetylene (C2H2) depositing gas 57 is/are utilized by the ion source of FIGS. 1-3. The silane and/or acetylene depositing gas passes through the gap at 63 between anode 25 and cathode 5. Unfortunately, certain of the elements in acetylene and/or silane gas is/are insulative in nature (e.g., carbide may be an insulator in certain applications). Insulating deposits resulting from the depositing gas can quickly build up on the respective surfaces of anode 25 and/or cathode 5 proximate gap 63. This can interfere with gas flow through the gap or slit, and/or it can reduce net current thereby adversely affecting the electric field potential between the anode and cathode proximate slit 15. In either case, operability and/or efficiency of the ion beam source is adversely affected. In sum, the flow of gas which produces a substantial amount of insulative material buildup in electrical gap 63 between the anode and cathode may be undesirable in certain applications.
In view of the above, it will be apparent to those skilled in the art that there exists a need for an ion source including a more efficient gas flow design.